sanitycheck: add ability to use arbitrary report for comparison

This adds the ability for sanitycheck to use arbitrary report file
for size comparison. This is useful when optimizing binary size.
For example, a report can be generated with master head, and used
as base for comparison.

Change-Id: Id8c61146c498ba62c4cc77a9ef7d390c0fb11487
Signed-off-by: Daniel Leung <daniel.leung@intel.com>